In a Nb3Al superconducting multifilament wire material produced by a rapid-heating and quenching process, as a barrier material for Nb3Al superconducting filaments, there has been used Nb which is a high melting-point material having excellent ductility and having a melting point of higher than 2,000° C. However, Nb has a critical temperature of about 9 K and exhibits superconductivity in an environment at extremely low temperatures, and therefore induces magnetic coupling of the filaments. On the other hand, for suppressing such magnetic coupling, an attempt has been made to use Ta which is a high melting-point material having low superconductivity. However, Ta has poor ductility and hence increases a risk of wire breakage during the wire drawing processing.
The matrix material used in the Nb3Al superconducting multifilament wire material produced by a rapid-heating and quenching process is limited to Nb or Ta which is a high melting-point metal because of the special ultrahigh temperature treatment conducted for the wire material. For this reason, there have been only two types of Nb3Al superconducting multifilament wire materials produced by a rapid-heating and quenching process, i.e., an all-Nb barrier filament wire material in which, as shown in FIG. 1, Nb3Al superconducting filaments having a Nb barrier are disposed around a dummy core formed from Nb, and an all-Ta barrier filament wire material in which, as shown in FIG. 2, Nb3Al superconducting filaments having a Ta barrier are disposed around a dummy core formed from Nb or Ta.
The all-Nb barrier filament wire material has excellent processability; however, when used in an environment at extremely low temperatures, as mentioned above, the superconductivity of Nb causes the filaments to be magnetically coupled with each other, that is, a so-called filament coupling phenomenon occurs. Therefore, the all-Nb barrier filament wire material behaves like a single bulk of wire, though it has a very fine multifilament structure, so that an unstable phenomenon called flux jump due to an increase of the magnetization occurs, thus causing an increase of the magnetic hysteresis loss.
As a new wire material for suppressing coupling of the filaments caused due to the superconductivity of Nb, the all-Ta barrier filament wire material using Ta, which is a high melting-point material having low superconductivity, as a barrier material has been developed. This wire material suppresses magnetic coupling of the filaments; however, the wire material has poor wire drawing processability, as compared to the wire material using a Nb barrier, and therefore, as mentioned above, another problem arises in that the risk of wire breakage during the wire drawing processing is increased.
For solving the above problems, studies have been made on a method of adding an alloy to the Nb barrier so that the Nb barrier has normal conductivity, and a method for improving the wire drawing processability of the Ta matrix by controlling the purity of or annealing conditions for a Ta rolled sheet used as a Ta barrier. However, any of these methods cannot achieve satisfactory wire drawing processability, and the problems have not yet been fundamentally solved.    JP-A-2002-33025    JP-A-2006-85555    K. Tsuchiya, T. Takeuchi, N. Banno et al., “Study of Nb3Al Wires for High-field Accelerator Magnet Applications”, IEEE Trans. Appl. Supercond., vol. 20, pp. 1411-1414, 2010.    N. Banno, T. Takeuchi et al., “Minimization of the hysteresis loss and low-field instability in technical Nb3Al conductors”, Supercond. Sci, Technol., vol. 21, 115020 (7 pp), 2008    T. Takeuchi, A. Kikuchi, N. Banno et al., “Status and perspective of the Nb3Al development”, Cryogenics, vol. 48, pp. 371-380, 2008